1. Field of the Invention
The present invention relates to coating method and apparatus, in particular to coating method and apparatus for use in manufacturing photosensitive materials and magnetic recording media.
2. Description of the Related Art
Photosensitive materials and magnetic recording media are manufactured through a coating step of forming a coating film by applying a prescribed coating liquid, such as magnetic liquid, to a continuously running sheet substrate strip (hereinafter referred to as xe2x80x9cwebxe2x80x9d). In recent years, magnetic recording media such as magnetic recording tape particularly for broadcasting or computer use have been rapidly improved in capacity and recording density, and there has been needed a coating technology capable of providing a magnetic layer which is extremely thin and of which surface is smooth.
Coating apparatus for applying a coating liquid to the surface of a web include, for example, apparatus of types such as roll coater, gravure coating, roll coating plus doctor roll, extrusion coating, and slide coating; in recent years, apparatus of extrusion type have been commonly used.
One method which employs an extrusion type of apparatus and in which the tip of a coating head is pushed against a web, as described in Japanese Patent Application Publication No. 58-109162, has been commonly used in the field of manufacturing magnetic recording media, since a thin and uniform coating layer can be obtained just by increasing the fluid pressure at the tip of the coating head utilizing the web tension, thereby eliminating the air accompanying the web. However, even in the method of applying a single coat of coating liquid using this type of coating head, the reduction in coating thickness is limited.
As a further method for forming a much thinner coating layer, there is a method in which a very thin coating layer can be formed by applying a coating liquid to a web to excess using a web-pushing type of extrusion coater and then scraping-off the excess coating liquid with a blade disposed on the downstream side of the extrusion coater, as described in Japanese Patent Application Publication No. 7-287843. In this case, the device which applies a coating liquid to a web to excess is not necessarily limited to the web-pushing type of extrusion coater, but coaters such as roller coater type coater, gravure coater and extrusion coater equipped with a back-up roll can be used.
In the method using the conventional scraping-off type of extrusion coater shown in FIG. 8, however, since the coating liquid recovered by scraping-off, with a blade 6, the excess amount of coating liquid having been applied to a web 5 at a coating head 3 has been once exposed to the atmosphere, the solvent contained in the coating liquid has been volatilized, thereby changes are caused in the fluid physical properties such as viscosity and solid content concentration. Thus, the method has a disadvantage in that, since the use of the coating liquid, as it is, of which viscosity has been changed adversely affects the amount of the coating liquid to be scraped-off or the final thickness of the coating film after being scraped-off with the blade 6, the recovered coating liquid must undergo treatment for adjusting its physical properties such as viscosity and solid content concentration in a coating liquid adjusting tank 8 before reused. In addition, since the recovering line for recovering the coating liquid is opened to the atmosphere, dust from the outside of the recovering line is likely to be included in the recovered coating liquid. Thus, the method also has a disadvantage in that dust cannot be fully removed even if the recovered coating liquid is filtered through a filter 9 when pumping the recovered coating liquid from a supply tank 1 to the coating head 3 with a pump 2.
The present invention has been made in the light of such circumstances, and an object of the present invention is to provide coating method and apparatus which enable the reuse of the coating liquid having been scraped-off from a web without doing any one of the coating liquid adjusting treatment and the filtering treatment, while maintaining the features of the scraping-off type of extrusion coater, which is suitable for obtaining a uniform and extremely thin coating film layer.
Another object of the invention is to provide a coating apparatus which enables the obtaining of a uniform and extremely thin coating film layer equal to or superior to that provided by the scraping-off type extrusion coater equipped with a doctor blade and enables the reuse of the coating liquid having been recovered from a web without doing any one of the coating liquid adjusting treatment and the filtering treatment.
In order to accomplish the above-described object, the present invention is directed to a coating method, comprising the steps of: pre-coating one side of a continuously running sheet substrate strip with a coating liquid, using a pre-coating device, in an amount in excess of that capable of obtaining an intended final coating thickness; and then scraping-off an excess amount of coating liquid with a scraping-off device provided on a downstream side along a running direction of the substrate relative to the pre-coating device, wherein the coating liquid having been applied to the substrate is not opened to the atmosphere during a time from the pre-coating until the scraping-off.
According to the present invention, the coating liquid having been scraped-off from the continuously running substrate strip can be reused without doing any one of the coating liquid adjusting treatment and the filtering treatment, while maintaining the features of the scraping-off type of extrusion coater, which is suitable for obtaining a uniform and extremely thin coating film layer, since the coating liquid having been applied to the substrate is not opened to the atmosphere during the time from the pre-coating of the substrate with the coating liquid until the scraping-off of the express amount of the coating liquid.
The term xe2x80x9cthe coating liquid having been applied to the substrate is not opened to the atmospherexe2x80x9d herein used means the state in which at least the surface of the substrate to which the coating liquid has been applied is not opened to the atmosphere, and it does not matter whether the lateral edge portions of the substrate are opened to the atmosphere or not. And this is the same in the following.
In an aspect of the present invention, the coating liquid having been scraped-off is recovered through a recovering line as a closed system and the recovered coating liquid is reused. Allowing the recovering line to be a closed system enables the fluid physical properties, such as viscosity and solid content concentration, of the coating liquid to be reused to be much less changed and dust and the like to be not included in the coating liquid.
In an aspect of the present invention, a desired coating thickness is obtained by adjusting a fluid resistance of the recovering line. Changing the fluid resistance of the recovering line enables the coating thickness to be varied. This case also embraces one form of the method in which the adjustment of the fluid resistance of the recovering line is combined with at least any one of the adjustment of the coating speed, the adjustment of the coating liquid viscosity and the adjustment of the solid content concentration.
In an aspect of the present invention, a desired coating thickness is obtained by adjusting at least one of the amount of the coating liquid to be used for the pre-coating and an amount of coating liquid to be scraped-off. This method enables a desired coating thickness to be obtained in a simple and easy manner.
In an aspect of the present invention, the amount of the coating liquid used for the pre-coating and the amount of the coating liquid scraped-off are measured with flowmeters, respectively, and at least one of the amounts is adjusted to allow a difference between the two measured amounts to provide the desired coating thickness. This method enables desired coating thickness to be obtained with a high accuracy.
In an aspect of the present invention, the desired coating thickness is obtained by adjusting a running speed of the substrate. Adjusting the running speed of the substrate (i.e., a coating speed) enables the coating thickness to be varied.
In an aspect of the present invention, the desired coating thickness is obtained by adjusting a viscosity of the coating liquid. Adjusting the viscosity of the coating liquid enables the coating thickness to be varied. This case also embraces one aspect of the method in which the adjustment of the coating liquid viscosity is combined with the adjustment of the coating speed.
In an aspect of the present invention, the desired coating thickness is obtained by adjusting a solid content concentration of the coating liquid. Changing the solid content concentration of the coating liquid enables the coating thickness to be varied. This case also embraces one aspect of the method in which the adjustment of the solid content concentration is combined with at least either one of the adjustment of the coating speed and the adjustment of the coating liquid viscosity.
In an aspect of the present invention, the scraping-off device is constructed so that the scraping-off device scrapes-off the excess amount of coating liquid by allowing a scraping-off portion thereof and the substrate to be pushed relative to each other, thereby pressing the coating liquid which has been applied to the substrate, the desired coating thickness being obtained by varying a pressing force applied to the coating liquid. This case also embraces one aspect of the method in which the adjustment of the pressing force applied to the coating liquid is combined with at least any one of the adjustment of the fluid resistance of the recovering line, the adjustment of the solid content concentration of the coating liquid, the adjustment of the coating liquid viscosity, the adjustment of at least either one of the amount of the coating liquid used for pre-coating and the amount of the coating liquid scraped-off and the adjustment of the running speed of the sheet substrate strip.
In an aspect of the present invention, the pressing force is obtained by lapping the substrate over the scraping-off portion and varied by adjusting a tensile force of the substrate exerted in the running direction of the substrate.
In an aspect of the present invention, the pressing force is obtained by arranging a back-up roller on the back side of the substrate, to which no coating liquid is applied, in such a manner as to face the scraping-off portion and is adjusted by adjusting the clearance between the scraping-off portion and the back-up roller.
In an aspect of the present invention, the pressing force is obtained by arranging a back-up roller made of an elastic material on the back side of the substrate, to which no coating liquid is applied, in such a manner as to face the scraping-off portion and is adjusted by adjusting at least one of a longitudinal elasticity modulus and a hardness of the back-up roller.
In an aspect of the present invention, the pressing force is obtained by arranging a spraying device which sprays fluid on the back side of the substrate, to which no coating liquid is applied, in such a manner as to face the scraping-off portion and is adjusted by adjusting a spraying pressure of the spraying device.
In an aspect of the present invention, the coating liquid is a magnetic coating liquid with magnetic particles dispersed therein. The coating method of the present invention is extremely effective in coating the substrate with a magnetic coating liquid which requires accuracy in the adjustment of the physical properties such as viscosity and solid content concentration.
In order to accomplish the above-described object, the present invention is also directed to a coating apparatus, comprising: a pre-coating device which pre-coats one side of a continuously running sheet substrate strip with a coating liquid in an amount in excess of that capable of obtaining an intended final coating thickness; a scraping-off device which scrapes-off an excess amount of coating liquid, the scraping-off device being arranged on a downstream side along a running direction of the substrate relative to the pre-coating device; and a closing mechanism which prevents the coating liquid having been applied to the substrate from being opened to the atmosphere between the pre-coating device and the scraping-off device.
According to the present invention, the coating liquid having been scraped-off from the continuously running substrate strip can be reused without doing any one of the coating liquid adjusting treatment and the filtering treatment, while maintaining the features of the scraping-off type of extrusion coater, which is suitable for obtaining a uniform and extremely thin coating film layer, since a mechanism is provided for preventing the coating liquid having been applied to the substrate from being opened to the atmosphere during the time from the pre-coating of the substrate with the coating liquid until the scraping-off of the excess amount of the coating liquid.
In an aspect of the present invention, a pre-coating portion of the pre-coating device and a scraping-off portion of the scraping-off device are integrated in a coating head; the closing mechanism is formed by allowing a lip surface of the coating head and the substrate to be adjacent to each other; and the pre-coating portion is arranged on the upstream side of the coating head, and the scraping-off portion is arranged on the downstream side of the coating head. The closing mechanism thus formed and constructed enables the coating liquid having been applied to the substrate not to be opened to the atmosphere during the time from the pre-coating of the substrate with the coating liquid until the scraping-off of the express amount of the coating liquid, since the coating liquid applied to the substrate to excess at the pre-coating portion is moved to the scraping-off portion, where the excess amount of the same is scraped-off, in such a state that it is sandwiched between the substrate and the lip surface.
In an aspect of the present invention, the pre-coating portion is provided with a coating slit for discharging the coating liquid; and the scraping-off portion is provided with a recovering slit for recovering the coating liquid having been scraped-off. Thus, all the paths of the pre-coating device and the scraping-off device can be a closed system.
In an aspect of the present invention, the coating apparatus further comprises: a pair of guide rollers which laps the substrate over the lip surface of the coating head, the pair of guide rollers being arranged on upstream and downstream sides relative to the coating head, wherein the desired thickness is obtained by adjusting a tensile force of the substrate exerted in the running direction of the substrate to press the coating liquid having been applied to the substrate.
In an aspect of the present invention, the coating apparatus further comprises: a back-up roller which is arranged on a back side of the substrate, to which no coating liquid is applied, in such a manner as to face the lip surface of the coating head, wherein the desired thickness is obtained by adjusting a clearance between the lip surface and the back-up roller to press the coating liquid having been applied to the substrate.
In an aspect of the present invention, the coating slit is in communication with a supplying line for supplying the coating liquid to be used for pre-coating; the recovering slit is in communication with a recovering line for recovering the coating liquid having been scraped-off; the supplying line and the recovering line are provided with flowmeters to measure the amount of the coating liquid used for pre-coating and the amount of the coating liquid having been scraped-off, respectively; and a flow control device is provided which controls at least one of a flow of the supplying line and a flow of the recovering line to allow a difference between the measured flows to provide the desired coating thickness. This coating apparatus enables the desired amount of the coating liquid to be applied to the substrate with a high accuracy.
In an aspect of the present invention, at least one of a doctor lip surface between the coating slit and the recovering slit and a recovering lip surface for scraping-off the excess amount of coating liquid, both constituting the lip surface of the coating head, has a shape of one of a circular arc projecting toward the substrate, a combined surface of at least two planes successively provided and forming a vertex projecting toward the substrate, and a combined surface of a circular arc curvature projecting toward the substrate and a plane. This enables the fluid pressure at the lip surface of the coating head to be high. Thus, the air accompanying the substrate is eliminated, and thereby a thin and uniform coating layer can be obtained.
In an aspect of the present invention, a relation R2 greater than R3 holds, where R2 represents a radius of curvature of the doctor lip surface and R3 represents a radius of curvature of the recovering lip surface. This enables the fluid pressure generated at the recovering lip surface to be high, thereby enabling the coating liquid to be easily scraped-off.
In an aspect of the present invention, the scraping-off portion is provided with a rotating rod which comes in contact with the surface of the substrate to which the coating liquid has been applied and scrapes-off the excess amount of coating liquid. Thus, when foreign matter is trapped between the rotating rod and the substrate, the foreign matter is easily eliminated, which results in reduction of streak defect.
In an aspect of the present invention, a pre-coating portion of the pre-coating device and a scraping-off portion of the scraping-off device are separately formed; and the closing mechanism comprises a covering member which covers at least the surface of the substrate to which the coating liquid has been applied, the covering member being arranged along a running route of the substrate between the pre-coating portion and the scraping-off portion. This enables the coating liquid having been applied to the substrate not to be opened to the atmosphere during the time from the completion of the pre-coating of the substrate with the coating liquid until the scraping-off of the express amount of the coating liquid, since the coating liquid applied to the substrate at the pre-coating portion is moved to the scraping-off portion, where the excess amount of the same is scraped-off, in such a state that it is sandwiched between the substrate and the covering member. Accordingly, it can be constructed in such a manner as to enable the coating liquid having been applied to the substrate not to be opened to the atmosphere even when the pre-coating portion of the pre-coating device and the scraping-off portion of the scraping-off device are formed separately.
In an aspect of the present invention, the pre-coating portion is provided with a coating slit for discharging the coating liquid; the scraping-off portion is provided with a recovering slit for recovering the coating liquid having been scraped-off; and a back-up roller is provided in such a manner as to face both of the pre-coating portion and the scraping-off portion. This enables all the paths of the pre-coating device and the scraping-off device to be a closed system even when the pre-coating portion and the scraping-off portion are separately formed, since providing the back-up roller and the guide member allows the pre-coating portion and the scraping-off portion to be constructed as if they were integrally formed.
In order to accomplish the above-described object, the present invention is also directed to a coating method, comprising the steps of: pre-coating one side of a continuously running sheet substrate strip with a coating liquid, using a pre-coating device, in an amount in excess of that capable of obtaining an intended final coating thickness; and then sucking-up an excess amount of coating liquid using a suction device provided on a downstream side along a running direction of the substrate relative to the pre-coating device, wherein the coating liquid having been applied to the substrate is not opened to the atmosphere during a time from the pre-coating until the sucking-up.
According to the present invention, one side of the continuously running sheet substrate strip is pre-coated with the coating liquid, using the pre-coating device, in the amount in excess of that capable of obtaining an intended final coating thickness and then the excess amount of the coating liquid is sucked-up using the suction device provided on the downstream side along the running direction of the substrate relative to the pre-coating device. As described so far, since the coating method of the present invention forcibly sucks-up the excess amount of the coating liquid using the suction device, the thickness of the coating liquid applied to the substrate can be controlled to a desired coating thickness with a higher accuracy compared with the conventional scraping-off type of method which employs a doctor blade. Furthermore, the coating liquid having been sucked-up from the continuously running sheet substrate strip can be reused without doing any one of the coating liquid adjusting treatment and the filtering treatment, since the coating liquid having been applied to the substrate is not opened to the atmosphere during the time from the completion of the pre-coating of the substrate with the coating liquid until the sucking-up of the excess amount of the coating liquid. Thus, a uniform and extremely thin coating film layer can be obtained which is equal to or superior to that obtained by the scraping-off type of method and the recovered coating liquid can be reused without doing any one of the coating liquid adjusting treatment and the filtering treatment. In this case, in order to obtain the above-described desired coating thickness, preferably at least any one of the running speed of the substrate, and the viscosity and the solid content concentration of the coating liquid is adjusted. Preferably the coating liquid is a magnetic coating with magnetic particles dispersed therein.
In an aspect of the present invention, a sucking force of the suction device is generated with a pump. In this method, the sucking force can be controlled easily by adjusting the driving speed of the pump.
In an aspect of the present invention, the coating liquid having been sucked-up with the suction device is recovered through a recovering line as a closed system and the recovered coating liquid is reused. Forming the recovering line as a closed system enables the fluid physical properties, such as viscosity and solid content concentration, of the coating liquid reused to be much less changed and makes it hard for dust and the like to be included into the coating liquid.
In an aspect of the present invention, a wet thickness of the coating liquid applied to the substrate through the pre-coating device is 3 mL/m2 or more. On the substrate from which the excess amount of the coating liquid has been sucked-up, a satisfactory coating film surface, which is free from streak defect, is likely to be formed.
In an aspect of the present invention, the desired coating thickness is obtained by adjusting at least one of the amount of the coating liquid to be used for pre-coating and an amount of the coating liquid to be sucked-up. This coating method enables the desired coating thickness to be obtained with a high accuracy.
In an aspect of the present invention, the amount of the coating liquid used for pre-coating and the amount of the coating liquid sucked-up are measured with flowmeters, respectively, and at least one of the amounts is adjusted to allow a difference between the two measured amounts to provide the desired coating thickness. This coating method enables the desired coating thickness to be obtained with a high accuracy. In this case, the adjustment is preferably conducted using a pair of pumps: a suction pump as a suction device; and a supply pump for supplying the coating liquid.
In order to accomplish the above-described object, the present invention is also directed to a coating apparatus, comprising: a pre-coating device which pre-coats one side of a continuously running sheet substrate strip with a coating liquid in an amount in excess of that capable of obtaining an intended final coating thickness; a suction device which sucks-up an excess amount of coating liquid, the suction device being arranged on a downstream side along a running direction of the substrate relative to the pre-coating device; and a closing mechanism which prevents the coating liquid having been applied to the substrate from being opened to the atmosphere between the pre-coating device and the suction device.
According to the present invention, one side of the continuously running sheet substrate strip is pre-coated with the coating liquid, using the pre-coating device, in the amount in excess of that capable of obtaining an intended final coating thickness and then the excess amount of the coating liquid is sucked-up using the suction device provided on the downstream side along the running direction of the substrate relative to the pre-coating device. As described so far, since the coating apparatus of the present invention forcibly sucks-up the excess amount of the coating liquid using the suction device, the amount of the coating liquid applied to the substrate can be controlled to a desired coating amount with a higher accuracy compared with the conventional scraping-off type of method which employs a doctor blade. Furthermore, the coating liquid having been sucked-up from the continuously running sheet substrate strip can be reused without doing any one of the coating liquid adjusting treatment and the filtering treatment, since the closing mechanism is provided for preventing the coating liquid having been applied to the substrate from being opened to the atmosphere during the time from the completion of the pre-coating of the substrate with the coating liquid until the sucking-up of the excess amount of the coating liquid.
In an aspect of the present invention, the sucking force of the suction device is generated with a metering pump. The sucking force can be controlled easily by adjusting the driving speed of the pump, and moreover, the use of the metering pump enables the control of the sucking force to be more accurate.
In an aspect of the present invention, a pre-coating portion of the pre-coating device and a suction portion of the suction device are integrated in a coating head; the closing mechanism is formed by allowing a lip surface of the coating head and the substrate to be adjacent to each other; and the pre-coating portion is arranged on the upstream side of the coating head, and the suction portion is arranged on the downstream side of the coating head. This enables the coating liquid having been applied to the substrate not to be opened to the atmosphere during the time from the completion of the pre-coating of the substrate with the coating liquid until the sucking-up of the excess amount of the coating liquid, since the coating liquid applied to the substrate at the pre-coating portion is moved to the suction portion, where the excess amount of the same is scraped-off, in such a state that it is sandwiched between the substrate and the lip surface.
In an aspect of the present invention, the pre-coating portion is provided with a coating slit for discharging the coating liquid; and the suction portion is provided with a recovering slit for recovering the coating liquid having been sucked-up. Thus, all the paths of the pre-coating device and the suction device can be a closed system.
In an aspect of the present invention, the coating apparatus further comprises a pair of guide rollers which laps the substrate over the lip surface of the coating head, the pair of guide rollers being arranged on upstream and downstream sides relative to the coating head.
In an aspect of the present invention, the coating apparatus further comprises a back-up roller which makes the substrate adjacent to the lip surface of the coating head, the back-up roller being arranged on a back side of the substrate, to which no coating liquid is applied.
In an aspect of the present invention, the coating slit is in communication with a supplying line for supplying the coating liquid to be used for pre-coating; the recovering slit is in communication with a recovering line for recovering the coating liquid having been sucked-up; the supplying line and the recovering line are provided with flowmeters to measure the amount of the coating liquid used for pre-coating and the amount of the coating liquid having been sucked-up, respectively; and a flow control device is provided which controls at least one of a flow of the supplying line and a flow of the recovering line to allow a difference between the measured flows to provide the desired coating thickness. This coating apparatus enables the desired amount of the coating liquid to be applied to the substrate with a high accuracy.
In an aspect of the present invention, a pre-coating portion of the pre-coating device and a suction portion of the suction device are separately formed; and the closing mechanism comprises a covering member which covers at least the surface of the substrate to which the coating liquid has been applied, the covering member being arranged along a running route of the substrate between the pre-coating portion and the suction portion. Thus, since the coating liquid having been applied to the substrate at the pre-coating portion is moved to the suction portion, where the excess amount of coating liquid is sucked-up, in such a state that it is sandwiched between the substrate and the covering member, the coating liquid having been applied to the substrate is not opened to the atmosphere between the pre-coating portion and the suction portion. Accordingly, this coating apparatus enables the coating liquid having been applied to the substrate not to be opened to the atmosphere even when the pre-coating portion and the suction portion are separately formed.
In an aspect of the present invention, the pre-coating portion is provided with a coating slit for discharging the coating liquid; the suction portion is provided with a recovering slit for recovering the coating liquid having been sucked-off; and a back-up roller is provided in such a manner as to face both of the pre-coating portion and the suction portion. This enables all the paths of the pre-coating device and the suction device to be a closed system even when the pre-coating portion and the suction portion are separately formed, since the back-up roller and the guide member allows the pre-coating portion and the suction portion to be constructed as if they were integrally formed.
The substrate used in the present invention may include the one on which a primer has been applied and set in advance.